1. Field of the Invention
The present invention relates to an ultrasound enhanced drug delivery catheter, and more particularly, to an ultrasound enhanced drug delivery catheter with a plurality of ultrasound elements.
2. Description of Related Art
Thrombus formation is a protective and healing mechanism, however, formation of thrombi can be detrimental. For instance, if a blood vessel becomes blocked, distal tissue may be deprived of oxygen with resulting damage or necrosis. In the case of cerebral circulation, an arterial thrombus blockage is one cause of cerebral strokes. In the case of coronary thrombosis, blockage and subsequent distal tissue necrosis of cardiac muscle tissue will impair cardiac pump output, may cause electrical abnormalities, and potentially catastrophic heart failure and death. The thrombus can form at the site of artery narrowing due to arterial wall damage or disease, or the thrombus may have broken free from some proximal site only to become wedged in a distal stenosis. Thrombus can also form subsequent to attempts to remove a stenosis using balloon angioplasty or rotary atherectomy.
Ultrasound catheters have been described specifically for removal or dissolution of thrombus (U.S. Patents: Tachibana U.S. Pat. No. 5,197,946; Bernstein U.S. Pat. No. 5,163,421; Weng U.S. Pat. No. 5,269,297). The catheters of Bernstein and Weng place an ultrasound generator external to the body and transmit acoustic energy through a metal wire wave-guide to the distal catheter. The catheter of Tachibana includes a small ultrasound element positioned at the distal end of the catheter that is energized by electrical wires. In either case, ultrasound energy is delivered to and radiated from the distal tip of the catheter in the vicinity of a blocking thrombus. The application of ultrasound can directly emulsify nearby thrombus through the motion of the catheter tip and associated cavitation.
The application of ultrasound can also enhance delivery of drug into a vessel wall. There are instances where the vessel wall is diseased or has been injured such as balloon angioplasty or rotary atherectomy. Narrowing of the vessel can occur in response to these injuries. Certain drugs, such as heparin, may inhibit this narrowing of the blood vessel if the drug can be delivered into the blood vessel wall. A catheter can be used to deliver drugs into any portion of the body or target organ. Ultrasound energy in the presence of these drugs can enhance the delivery through and across bodily fluids and tissue. Hence, an ultrasound drug delivery catheter placed in a blood vessel will assist delivery across the blood vessel wall, whether it be an artery or a vein, into the surrounding muscle or tissue.
The intensity of the ultrasound delivered from an ultrasound element decreases exponentially with radial distance from the catheter tip. Hence, treatment of thrombi is limited to a few millimeters of the catheter tip of a catheter with an ultrasound element. This small treatment area may be effective for small volume clots, however, larger clots must be treated one section at a time.
Some thrombi can be large. For instance, a deep vein thrombus in a patient's lower leg and can have a length from several centimeters to as much as 12 to 20 inches long. Early treatment protocols for these long thrombi used a drug infusion catheter to drip lytic drug at one end of a thrombus. As the thrombus was dissolved, the catheter would be advanced. This process was repeated until the entire clot was dissolved. More current therapy for a deep vein thrombosis is to use an infusion catheter with drug infusion ports distributed along the lateral dimension of the catheter. The catheter can be pushed through the entire length of the clot. The thrombolytic drug is then infused throughout the lesion for a period of hours.
There is a need for an ultrasound catheter that is useful for treating a deep vein thrombus to enhance and accelerate the action of the thrombolytic drug. There is a further need for an ultrasound catheter that is useful for treating vessel lesions, particularly those that have extensive lengths.